It is well known that gas is generated inside a case of a secondary battery such as a lithium-ion battery during charge in a manufacturing process of the secondary battery.
Therefore, the manufacturing process of the secondary battery includes a step of releasing the gas generated in the case of the secondary battery by disposing the secondary battery in a reduced pressure environment in many cases as described in below-described Patent Document 1.
As this step of venting the gas in the secondary battery, the secondary battery is disposed in the reduced pressure environment to vent the gas and then is returned into a normal pressure environment and an opening for venting the gas is completely sealed (actual sealing) in the conventional art.